Volume 9, Number 5
Sep 1985

DTPA, a Chelate That Helps Two Ways

A chelate, or more properly, a chelating agent, is a compound
like EDTA, chlorophyll or hemoglobin that has niches in its ring
structure for heavy metals like iron and copper. It attracts and
holds the metal ions so tightly that they lose their normal
characteristics, such as the ability to catalyze oxidation or form a
precipitate. The process of attracting and holding is sometimes
called "sequestration."

Anything that can prevent oxidation is sure to be used as a
preservative, and such is indeed the case here. In the food industry
sequestering agents aid in stabilizing color, flavor and
texture.

DTPA, a chelating agent useful for the manufacture and
preservation of paper, has been the subject of investigation in two
disparate settings recently: a conservation research laboratory in
Russia, and the lab of a commercial supplier of chelates in
America.

The first research was reported in the Preprints of
last September's ICOM meeting in Copenhagen.1 Newsprint, offset
paper and 100% rag paper were both made and treated in different
ways, then heat aged in closed glass tubes to show the effects of
deacidification, tap vs. Distilled water, and DTPA alone and in
combination with other conditions. Strength (folding endurance),
color (brightness) and pH were all measured at 3-day intervals.
DTPA, even when used alone, had a startlingly beneficial effect on
all three kinds of paper. One would of course expect DTPA to protect
newsprint, which deteriorates mainly by oxidation, but look at the
results for 100% cellulose (sometimes called "rag") paper in Graph
0:

Fig. 1. Logarithm of the number of double folds
(1g N) vs. time of accelerated aging (T).

___

Untreated

----

Treated with DTPA solution

*

Measurement made under load 3.9 N

**

Measurement made under load 6.9 N

When used together with calcium and magnesium bicarbonate in
solution, it made the paper four times as strong after 12 days in
the oven as it would have been after deacidifying alone. It also
kept the pH 1½ points higher than the pH for paper that was only
deacidified. Here is the graph showing the effect of the various
treatments on strength and pH:

Fig. 4. Changes in the properties of newsprint
subjected to various types of stabilization processes vs. time of
accelerated aging (T).

a - Logarithm of the number of double folds (1g N)
b - Water extract pH

The second report, which appeared in the June Tappi
Journal,2 describes a problem in the bleaching of pulp: heavy
metals from the wood itself, especially iron, copper and manganese,
rapidly break up the hydrogen peroxide bleach into oxygen and water
before it has a chance to do its job properly, which means that
larger quantities have to be used. This effect can be brought under
control by using a sodium salt of DTPA in combination with sodium
silicate and magnesium sulphate to precipitate the metal compounds
formed. (This is not news actually. The purpose of the article was
to give two case studies and some cost savings figures.)

DTPA stands for diethylenetriamine pentaascetic acid.

References

1. H.G. Blank and S. A. Dobrusina, "Raising of the Book-Paper
Longevity by Means of Chelates and Ca-Chelate Compound."
84.14.13-84.14.15. The authors are at the M. E. Saltykov-Shchedrin
State Public Library, 18 Sadovaja Street, 191069 Leningrad USSR.